1. Field of the Invention
The present invention is in the field of epoxy formulations to be used primarily as potting and sealing material in electronic components. More specifically, the present invention relates to an improved epoxy formulation that is adapted to be used in electronic components in conjunction with no-lead solder materials which require higher soldering temperatures than conventional lead-based solders.
2. Brief Description of Background Art
Epoxy formulations in the electronic industry are used for potting and sealing electronic components, such as resistors, resistor networks and trimming potentiometers. In the past the electronic industry used primarily lead-based solders, and prior art epoxy formulations functioned adequately when exposed to the soldering temperatures employed in conjunction with lead-based solders. However, the electronic industry is gradually moving away from using lead-based solders. Instead, the use of lead-free silver-copper-nickel-cobalt-bismuth based solders is gradually becoming the standard in the industry. Prior art epoxy formulations do not perform well when exposed to the higher (approximately 225xc2x0 C. to 275xc2x0 C.) temperatures required by the use of lead-free silver-copper-nickel-cobalt-bismuth based solders, because the conventional epoxy formulations tend to degrade and crack when exposed to these temperatures. In experiments performed comparing the novel epoxy formulation of the present invention with the conventional prior art epoxy formulations, the conventional prior art formulations often char, turn dark brown or black, become brittle, crack internally, separate from a component polymeric housing, from a ceramic substrate, and/or from a metal wire or other metal connector when processed at the higher temperatures (approximately 255xc2x0 C. to 275xc2x0 C.) required for the lead free solder re-flow, or after such process is followed by one or more thermal shock cycles (example: cooling to xe2x88x9265xc2x0 C. and warming +150xc2x0 C.). The conventional, prior art epoxy formulations also exhibit significant adhesive and/or cohesive cracks of the cured epoxy, after several infrared furnace solder re-flow cycles are followed by several thermal shocks between xe2x88x9265xc2x0 C. and +150xc2x0 C.
Attempts to provide epoxy formulations which would be compatible with the modern lead-free high-temperature solders by providing epoxy materials containing significant quantities of high temperature resins are, generally speaking, unsuccessful, because when such high temperature resins are used in the epoxy, then the epoxy becomes brittle and cracks at low (xe2x88x9265xc2x0 C.) temperature. Another disadvantage perceived in the prior art of using a high-temperature resin is that such resins require more aggressive catalysts than standard resins, and the shelf-life of a single part epoxy formulation containing such more aggressive catalysts, generally speaking, is too short to be acceptable. In this connection it will be readily understood by those skilled in the art that a single-part epoxy formulation is a formulation where the catalyst is mixed in with the formulation and no further mixing of a component or ingredient is required before the epoxy is employed for its intended use. This is to be contrasted with a two-part formulation, where one part contains the catalyst, the second part contains another ingredient or component essential for curing, and where the two parts have to be mixed for the curing process to begin.
Thus, there is a need in the state-of-the-art for a single part epoxy formulation to be used in electronic components and the like, which formulation is compatible with the use of modern lead-free solders requiring high (approximately 255xc2x0 C. to 275xc2x0 C.) temperatures for soldering, withstands cold and hot thermal shock cycles without significant deterioration, and which has an acceptable shelf-life. The present invention provides such an improved epoxy formulation.
The following United States and foreign patents or published patent applications serve as background to the present invention: U.S. Pat. No. 5,484,854; U.S. Pat. No. 5,049,596; U.S. Pat. No. 4,931,515; U.S. Pat. No. 4,701,481; U.S. Pat. No. 4,692,499; U.S. Pat. No. 4,631,230; U.S. Pat. No. 4,368,299; U.S. Pat. No. 4,328,150; U.S. Pat. No. 4,210,704; U.S. Pat. No. 4,137,275; U.S. Pat. No. 3,868,613; EPA 95100013.2; EPA 92113404.5; EPA 96301875.9; EPA 92200299.3 and EPA 87200018.7.
In accordance with the present invention an epoxy formulation or material is provided which is suitable for potting and sealing electronic components, and is compatible with the high (approximately 255xc2x0 C. to 275xc2x0 C.) temperature required for soldering modern lead-free solders. The epoxy material or epoxy formulation comprises approximately 2 to 13 per cent by weight of diglycidyl ether bisphenol A, a resin which is a low temperature resin (all percentages are by weight unless otherwise indicated), and approximately 40 to 70% phenol formaldehyde resin which is a high temperature resin. The formulation comprises the following further components in the following percentage ratios:
The above-enumerated components of the composition are thoroughly admixed at ambient temperature that preferably does not exceed 90xc2x0 C. Surprisingly, although the epoxy composition of the invention includes a more aggressive catalyst, it has a shelf-life of approximately 4 to 12 months. It is applied manually or by machinery to coat surfaces, fill cavities and serve for potting and sealing parts on electronic components. The epoxy composition is cured by exposure to heat in the temperature range of 120xc2x0 C. to 170xc2x0 C. for approximately 60 to 240 minutes. Again, surprisingly, although the composition of the invention includes a relatively high percentage of phenol formaldehyde (high temperature) resin, it is not only capable of withstanding the soldering temperatures of approximately 255xc2x0 C. to 275xc2x0 C., but it also withstands without significant deterioration several thermal shock cycles which include cooling to xe2x88x9265xc2x0 C. and heating to +150xc2x0 C.